1. Field of the Invention
The present invention relates to a die assembly such as a pressing die assembly for bending a blank to a desired shape or a trimming die assembly for drawing a blank and trimming a peripheral edge thereof, and a method of manufacturing such a die assembly.
2. Description of the Related Art
Automotive bodies are produced by pressing, drawing, and trimming blanks.
Die assemblies for pressing, drawing, and trimming blanks are generally made of cast iron or cast steel, and so rigid that they can withstand several hundred thousand pressing cycles. However, such die assemblies are expensive to manufacture.
Other die assemblies which do not machine blanks, but are relatively inexpensive to manufacture and suitable for manufacturing products of many different types in small quantities are made of base materials of zinc alloy, as disclosed in Japanese laid-open patent publications Nos. 5-84591, 5-195121, 5-208296, and 5-237656.
Specifically, Japanese laid-open patent publication No. 5-84591 discloses that a zinc alloy containing magnesium and aluminum and having a Vickers hardness of 150 or more is welded on a zinc alloy containing aluminum and copper by build-up welding.
Japanese laid-open patent publication No. 5-195121 proposes a zinc alloy for a pressing die assembly, which is made of 9.5-30 wt % of aluminum, 6.0-20 wt % of copper, 0.01-0.2 wt % of magnesium, and the remainder of zinc.
Japanese laid-open patent publication No. 5-237656 shows a method of repairing a die assembly of aluminum by plating only a peripheral region of the die assembly, except for a region to be repaired, with Nixe2x80x94P, and padding the region to be repaired with a filler metal for thereby achieving desired hardness of the peripheral region.
According to Japanese laid-open patent publication No. 5-208296, it has been proposed to use a zinc alloy as a base material of a die assembly for molding plastics and to use an aluminum alloy containing Si or the like as a filler metal for repairing the die assembly.
Die assemblies made of base materials of zinc alloy are lightweight, easy to cast, and of excellent maintainability. Though zinc alloys have excellent machinability, they are soft. Therefore, a different metal needs to be added to a certain region of zinc alloy if a cutting edge or the like is mounted on the zinc alloy.
Specifically, even if a zinc alloy containing magnesium and aluminum or an aluminum alloy containing Si or the like is welded on a zinc alloy containing aluminum and copper by build-up welding, as disclosed in Japanese laid-open patent publications Nos. 5-84591 and 5-208296, the welded region is not sufficiently hard for use as a trimming blade. The method disclosed in Japanese laid-open patent publication No. 5-237656 fails to achieve a sufficient level of hardness.
Therefore, die assemblies of zinc alloy are actually limited to use as die assemblies for molding plastics.
Japanese laid-open patent publication No. 5-195121 proposes a pressing die assembly, which has a cutting edge that needs to be hard and a bending member that needs to be resistant to wear. However, the problems of the cutting edge and the bending member remain to be solved.
According to other proposals, a cutting edge that needs to be hard is not formed by build-up welding, but a region to serve as a cutting edge is plated with a hard chromium layer or a cutting edge is formed by evaporation, sputtering, or the like. With these proposals, however, it is difficult to form a cutting edge of a thickness required to keep it durable. In addition, these proposed processes are not cost-effective enough.
Furthermore, as disclosed in Japanese patent No. 2838657, a cutting edge is formed by defining a bevel on an edge of a die assembly, welding a filler metal of high hardness on the bevel by build-up welding, and then grinding the filler metal with a grinder. However, it is known in the art that only a Cu-based or Zn-based material can be directly welded to a zinc alloy, but there is no Cu-based or Zn-based material that is hard enough for use as a cutting edge material.
The weldability of a zinc alloy and a nickel alloy with respect to each other is so poor that the nickel alloy cannot be welded on the zinc alloy to form a highly hard build-up welded region. As a result of studies made by the inventors of the present invention, it has been found that a copper alloy can be welded to both a zinc alloy and a nickel alloy. The present invention resides in that an underlying layer of copper alloy is welded on a base material and an overlying layer of nickel alloy is welded on the underlying layer.
A die assembly according to the present invention comprises an upper die and a lower die for trimming or bending a workpiece, at least one of the upper die and the lower die having a cutting edge or a bending member. The upper die and the lower die being made of a base material of an aluminum/copper-based zinc alloy, the cutting edge or the bending member having a machined build-up welded region comprising an underlying layer made of a filler metal of a copper-based material that can be welded to a zinc alloy and an overlying layer made of a filler metal of a nickel-based material that has a sufficient hardness and can be welded to the underlying layer of the copper-based material.
If the overlying layer is brought into contact with the base material when it is welded on the underlying layer, sputtering occurs, causing a welding defect. It is therefore necessary to weld the overlying layer on the underlying layer out of contact with the base material.
The at least one of the upper die and the lower die may have a bevel on which the cutting edge or the bending member is disposed. The bevel has a vertical dimension which substantially corresponds to the width of one weld pass of weld beads and a horizontal dimension which substantially corresponds to the width of two weld passes of weld beads, and including a flat area in a transversely outer region thereof, the flat area having a width which substantially corresponds to the width of one weld pass of weld beads. With this structure, the underlying layer is prevented from falling, and sputtering and blow holes are prevented from occurring due to contact between the base material and the overlying layer.
For effectively preventing blow holes from occurring, the bevel may have a chamfered surface and an extension extending therefrom. The underlying layer is disposed in covering relation to the bevel in its entirety and made of a copper-based material, and the overlying layer is disposed on the underlying layer out of contact with the base material and made of a nickel-based material. The overlying layer can be welded while a produced gas is being discharged through the underlying layer formed on the extension.
The copper-based material may be pure copper, aluminum bronze, silicon bronze, or the like. For better weldability, silicon bronze is most preferable.
The silicon bronze is preferably composed of 1.0-8.0 wt % of Si, 0.3-4.0 wt % of Mn, 0.03-4.5 wt % of Pb, 0.03-11.0 wt % of Al, 0.03-7.0 wt % of Ni, 0.03-6.0 wt % of Fe, and the remainder of Cu.
Si (silicon) is an element required for deoxidization, and is also an element for increasing hardness. If the amount of Si were less than 1.0 wt %, then deoxidization would be insufficient and blow holes would be liable to occur. If the amount of Si exceeded 8 wt %, then the silicon bronze would not be of a one-phase structure, but many phases would be precipitated, and the structure would become fragile.
Mn (manganese) is an element required for deoxidization and desulfurization. If the amount of Mn were less than 0.3 wt %, then the effect of its addition would not appear. If Mn were added in excess of 4.0 wt %, then no further effect would be achieved.
Pb (lead) is an element for increasing machinability. If the amount of Pb were less than 0.03 wt %, then almost no effect would be obtained from its addition. If the amount of Pb exceeded 4.5 wt %, then it would easily bring about weld cracks.
Al (aluminum) is a colorant. If Al increases, then the silicon bronze changes its color from copper red to gold. Al is also an element for increasing hardness. If the amount of Al were less than 0.03 wt %, then the effect of its addition would not appear. If the amount of Al exceeded 11 wt %, then the hardness and elongation would be lowered.
Ni (nickel) is an element effective to increase hardness. If the amount of Ni were less than 0.03 wt %, then almost no effect would be obtained from its addition. If the amount of Ni exceeded 7.0 wt %, then it would be excessive and the hardness would be lowered.
Fe (iron) is an element for reducing the grain size and increasing hardness. If the amount of Fe were less than 0.03 wt %, then almost no effect would be obtained from its addition. If the amount of Fe exceeded 6.0 wt %, then it would be excessive and no effect would be obtained from its addition.
The nickel-based material of the overlying layer preferably is composed of 1.0-6.0 wt % of B, 5.0-20.0 wt % of Cr, 1.0-7.0 wt % of Si, 0.03-4.0 wt % of Fe, 0.5-6.0 wt % of Cu, and the remainder of Ni.
B (boron) is an element for reducing the grain size and increasing hardness. If the amount of B were less than 1.0 wt %, then the effect of its addition would be extremely small. If the amount of B were in excess of 6.0 wt %, then it would be excessive, tending to produce weld cracks.
Cr (chromium) is an element for increasing hardness and increasing acid resistance at high temperatures. If the amount of Cr were less than 5.0 wt %, then the effect of its addition would be small. If the amount of Cr were in excess of 20.0 wt %, then it would be excessive, lowering the machinability.
Si (silicon) is a deoxidizing element and an element for improving fluidity. If the amount of Si were smaller than 1.0 wt %, then the effect of its addition for fluidity would be small. If the amount of Si were greater than 7.0 wt %, then it would be excessive, tending to produce weld cracks.
Fe (iron) is an element for reducing the grain size and increasing hardness. If the amount of Fe were less than 0.03 wt %, then almost no effect would be obtained from its addition. If the amount of Fe exceeded 4.0 wt %, then it would be excessive and no effect would be obtained from its addition.
Cu (copper) is an element effective for increasing toughness. If the amount of Cu were less than 0.5 wt %, then almost no effect would be obtained from its addition. If the amount of Cu exceeded 6.0 wt %, then it would be excessive, and the toughness would be lowered, tending to cause weld cracks.
For welding the layers on the bevel, a portion of the die along the bevel is preheated, and then an oxide film on the bevel is removed. Thereafter, the underlying layer is welded on the bevel. At least a portion of the die along the underlying layer is preheated, and an oxide film on the underlying layer is removed, after which the overlying layer is welded on the underlying layer. The weldability of the cutting edge is increased by thus preheating the base material in its entirety or a portion thereof where the layers are to be formed by build-up welding, before the layers of copper-based material and nickel-based material are formed by build-up welding.
The bevel is preheated to about 200xc2x0 C. before the underlying layer is welded on the bevel, and the underlying layer is preheated to about 250xc2x0 C. before the overlying layer is welded on the underlying layer.
The underlying and overlying layers should preferably be welded by a TIG (tungsten inert gas) welding process because the TIG welding process is less conducive to the generation of blow holes than MIG welding and arc welding processes.
The underlying layer may be welded by an AC TIG welding process. The AC TIG welding process has a cleaning action to remove an oxide film to make the underlying layer smooth. Specifically, the aluminum/copper-based zinc alloy is liable to produce an oxide film thereon which is responsible for a welding failure. According to the AC TIG welding process, a negative pole spot is apt to be formed in an area where an oxide is present on the surface of the base material. The negative pole spot removes the oxide with intensive heat, then moves toward a next oxide, and similarly removes the next oxide.
The AC TIG welding process is also effective to minimize the penetration of the underlying layer into the base material, and prevent a zinc alloy of the base material from rising to or nearly to the surface of the underlying layer, thereby preventing sputtering.
If the underlying layer were welded by the AC TIG welding process, then since the aluminum/copper-based zinc alloy has a low melting point, the base material might be melted before the welding rod is melted, producing a hole and causing a welding failure.
The overlying layer may be welded by a DC TIG welding process. The DC TIG welding process serves to increase the weldability. Specifically, since the underlying layer is made of a copper-based material which is a good heat conductor, the underlying layer does not easily reach its melting point. However, because the DC TIG welding process has a large current capacity and allows the overlying layer to penetrate deeply into the underlying layer, the underlying layer is melted for increased weldability.
Both the AC TIG welding process and the DC TIG welding process should preferably employ a shield gas of helium or a mixture of helium and argon. Since helium is more effective to concentrate heat without spreading it than argon, it is preferable to use a gas of helium or a mixed gas of helium and argon for TIG-welding materials of high heat conductivity, such as zinc alloy. It is preferable to weld the underlying layer according to the AC TIG welding process and to weld the overlying layer according to the DC TIG welding process.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.